1. Field of the Invention
This invention relates to internal combustion engines using variable camshaft timing.
2. Background Art
In conventional engines, the timing between the crankshaft and camshafts is rotationally fixed. Recently, engines have been improved by including mechanisms for automatically advancing or retarding camshaft rotation relative to the rotation of the crankshaft. By providing automatic advance or retard, it is possible to maximize fuel economy and minimize emissions in the engine""s exhaust. It is also possible to increase the peak torque and improve fuel economy by optimizing the phase angle relationship of the camshaft relative to the crankshaft.
Phase shift control is provided by a hydraulic coupler that rotationally couples the camshaft drive sprocket to a camshaft flange. An engine control data processor monitors continuously varying engine operating conditions and provides a control signal to the hydraulic coupler to set the desired relative phase angle for the engine operating conditions. Variable camshaft timing systems have been developed with the objective of correcting the air charge drop during camshaft retard that may result in a dip in torque response. One example of this approach is disclosed in U.S. Pat. No. 5,690,071 which discloses a method for adjusting the variable camshaft timing induced air variation that uses the air bypass valve to compensate for induced air charge variation. The patent also proposes the use of an electronically-controlled throttle that compensates for induced air change variation.
While this approach improves the performance of the variable camshaft timing system in normal circumstances, the use of a measured cam position and cam rate-of-change values is based on a signal that includes oscillations and a high degree of signal filtering. Signal filtering causes delays in response to the continuously varying engine operating conditions that may result in a reduction in torque response or increase in emissions.
To minimize delay caused by filtering measured signals, it has been proposed to use a model of the variable camshaft timing rate-of-change to control engine operation. Under cold-start conditions wherein the engine oil is cold and highly viscous, or if inadequate oil is available to the engine, the hydraulic device rotationally coupling the camshaft drive sprocket to the camshaft flange may be substantially retarded relative to the model. Under these operating conditions, the model of the variable camshaft timing rate-of-change if used to control an engine operating system would be less effective than the measured camshaft timing approach proposed in U.S. Pat. No. 5,690,071.
There is a need for an internal combustion engine having a variable camshaft timing system that minimizes delays caused by filtering oscillations in a measured camshaft position based system, but that includes a measured camshaft position system for controlling the camshaft position under cold-start or when low-oil pressure is encountered by the engine.
The present invention addresses the above problems and fulfills the need for a system that maximizes the benefits of variable camshaft timing as summarized below.
According to the present invention, a method of determining the position of a variable cam timing phasing system for an engine having an engine controller that controls the flow of intake and exhaust gases during the combustion process is provided. The method comprises the steps of creating a model for estimating a calculated cam position and a calculated rate-of-change of the cam position. The method also includes the step of measuring an actual cam position and an actual rate-of-change of the cam position. According to the method, the calculated cam position is compared to the measured cam position and the calculated rate-of-change of the cam position and measured actual rate-of-change of the cam position are blended based upon the comparison. The engine controller is adjusted dependent upon the blended rate-of-change of the cam position.
According to another aspect of the invention, an algorithm based upon test data and simulations is provided for estimating the calculated rate-of-change of the cam position.
Based upon another aspect of the invention, an algorithm has been developed for blending the estimated and measured actual cam positions and measurement based rate-of-change of the cam position.
According to yet another aspect of the invention, the engine controller may be an electronic throttle control. The engine may have a dual equal cam with equal phase intake and exhaust cams that are advanced and retarded equally relative to the crankshaft. Alternatively, the invention could be used in conjunction with an idle speed valve system and other forms of variable cam timing systems including exhaust only or dual independent cams.
The invention also relates to the engine controller using the calculated cam position and calculated rate-of-change of the cam position when the comparison of the calculated cam position to the measured cam position is below a threshold value. The engine controller uses the measured cam position and measured rate-of-change of the cam position when the comparison of the calculated cam position to the measured cam position is above a high threshold value. When the comparison of the calculated cam position to the measured cam position is above the low threshold value and below the high threshold value, the engine controller uses a blended rate-of-change based on the comparison and further based upon the calculated rate-of-change and measured rate-of-change of the cam position.
According to another aspect of the invention, a method of determining a blended rate-of-change of a cam position in an internal combustion engine having a variable phase cam timing system is proposed that includes a first step of determining a reference cam position. The next step of the method is to utilize a model to determine an estimated cam position and an estimated rate-of-change of the cam position. The system measures an actual cam position and determines a filtered rate-of-change of the actual cam position based upon the measurement. The model estimated cam position is compared to the measured actual cam position. The method further includes the step of determining the blended rate-of-change of the cam position based on the comparison, and further based on the estimated rate-of-change of the cam position and the filtered rate-of-change of the cam position.
The invention may also be seen as a system for determining the rate-of-change of a cam position for an internal combustion engine having a crankshaft and a camshaft. The internal combustion engine includes a variable phase cam timing system that alters a phase relationship between the camshaft and the crankshaft. The system comprises a data processor having a model for determining an estimated cam position relative to a reference cam position and also estimating a rate-of-change of the cam position. A sensor is provided for measuring an actual cam position and a data processor includes a filtering algorithm for determining the filtered rate-of-change of the actual cam position. The data processor compares the estimated cam position to the measured actual cam position and determines a blended rate-of-change of the cam position based upon the comparison, and further based on the estimated rate-of-change of the cam position and the filtered rate-of-change of the cam position.
These and other aspects of the present invention will be better understood in view of the attached drawings and detailed description of the invention.